This invention relates generally to the cryogenic rectification of feed air to produce oxygen and/or nitrogen, and is particularly useful for use in an integrated gasification combined cycle system.
The cryogenic rectification of feed air typically is carried out with a double column system wherein an initial separation is carried out in a higher pressure column and the final separation is carried out in a lower pressure column. The products are produced in the lower pressure column at slightly above ambient pressure. Three column systems are known which can produce oxygen and nitrogen at higher pressures, such as would be useful with a gas turbine system, but such heretofore known systems require a high power input. A three column cryogenic air separation system which can produce products with lower power requirements than heretofore available three column systems would be highly desirable.
Accordingly, it is an object of this invention to provide an improved cryogenic air separation system using three columns to produce oxygen and/or nitrogen which can operate with lower power requirements than heretofore available such systems.
It is another object of this invention to provide an improved three column cryogenic air separation system for use in an integrated gasification combined cycle system.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A cryogenic rectification method for producing at least one of oxygen and nitrogen comprising:
(A) passing a first feed air stream into a higher pressure column of a cryogenic rectification plant which also comprises a lower pressure column and a medium pressure column, and passing a second feed air stream into the medium pressure column, said second feed air stream being at a pressure which is less than the pressure of the first feed air stream;
(B) producing by cryogenic rectification oxygen-enriched liquid and nitrogen-enriched fluid within the higher pressure column;
(C) passing oxygen-enriched liquid into the medium pressure column and producing intermediate vapor and intermediate liquid by cryogenic rectification within the medium pressure column;
(D) passing a vapor stream taken from below the top of the higher pressure column in indirect heat exchange with intermediate liquid to produce higher pressure liquid and passing higher pressure liquid into the higher pressure column;
(E) passing fluid from the medium pressure column into the lower pressure column and producing nitrogen-richer fluid and oxygen-richer fluid by cryogenic rectification within the lower pressure column; and
(F) recovering at least one of the nitrogen-richer fluid and oxygen-richer fluid as product.
Another aspect of the invention is:
An apparatus for producing at least one of oxygen and nitrogen comprising:
(A) a cryogenic rectification plant comprising a higher pressure column, a lower pressure column, and a medium pressure column having a bottom reboiler;
(B) means for passing feed air into the higher pressure column;
(C) means for passing feed air into the medium pressure column;
(D) means for passing fluid from below the top of the higher pressure column into the medium pressure column bottom reboiler, and means for passing fluid from the medium pressure column bottom reboiler into the higher pressure column;
(E) means for passing fluid from medium pressure column into the lower pressure column; and
(F) means for recovering fluid as product from at least one of the upper portion of the lower pressure column and the lower portion of the lower pressure column.
As used herein, the term xe2x80x9ctrayxe2x80x9d means a contacting stage, which is not necessarily an equilibrium stage, and may mean other contacting apparatus such as packing having a separation capability equivalent to one tray.
As used herein, the term xe2x80x9cequilibrium stagexe2x80x9d means a vapor-liquid contacting stage whereby the vapor and liquid leaving the stage are in mass transfer equilibrium, e.g. a tray having 100 percent efficiency or a packing element height equivalent to one theoretical plate (HETP).
As used herein, the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen and nitrogen, such as ambient air.
As used herein, the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer""s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. The term, double column, is used to mean a higher pressure column having its upper portion in heat exchange relation with the lower portion of a lower pressure column. A further discussion of double columns appears in Ruheman xe2x80x9cThe Separation of Gasesxe2x80x9d, Oxford University Press, 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein, the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein, the term xe2x80x9creboilerxe2x80x9d means a heat exchange device that generates column upflow vapor from column liquid.
As used herein, the terms xe2x80x9cturboexpansionxe2x80x9d and xe2x80x9cturboexpanderxe2x80x9d mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration.
As used herein, the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d mean those sections of a column respectively above and below the mid point of the column.
As used herein, the term xe2x80x9cbottomxe2x80x9d when referring to a column means that section of the column below the column mass transfer internals, i.e. trays or packing.
As used herein, the term xe2x80x9cbottom reboilerxe2x80x9d means a reboiler that boils liquid from the bottom of a column. A bottom reboiler may be located within or outside of the column.
As used herein, the term xe2x80x9cintermediate reboilerxe2x80x9d means a reboiler that boils liquid from above the bottom of a column. An intermediate reboiler may be located within or outside of the column.
As used herein, the term xe2x80x9ctopxe2x80x9d when referring to a column means that section of the column above the column mass transfer internals, i.e. trays or packing.